Shrinking the size of the undo tablespace

-- Create a new undo tablespace with smaller size.
SQL> create undo tablespace UNDO_SMALL datafile ''undo-small.dbf' size
100m;
-- Set new tablespace as undo_tablespace.
SQL> alter system set undo_tablespace=UNDO_SMALL;
-- Drop the old tablespace.
SQL> drop tablespace UNDO_LARGE including contents;

The drop command may give ORA-30013 until existing transactions commit
or rollback.